A wheeled vehicle such as a bicycle or motorcycle is used to traverse a variety of terrain. These vehicles are designed to use a power source to drive a wheel or wheels through a power transmission system known as a drivetrain. A chain or belt driven drivetrain transfers rotary motion from the power source to the ground via a tractive or driving force between the wheel or wheels and the ground. It is this driving force that is responsible for powered acceleration of the vehicle. In some vehicles, for example a motorcycle, braking forces may also be transferred through the drivetrain to decelerate the vehicle.
Some wheeled vehicles have a suspension system that uses a spring and damper to isolate and control the movement of the vehicle's wheel(s) from the movement of its suspended mass (the suspended mass comprises the total sprung mass including the vehicle chassis and operator). A suspension system allows the suspended wheel(s) to move a distance known as the suspension travel, as the suspension is moved from a fully extended state to a fully compressed state. A suspension system may be designed so that a vehicle reacts to terrain undulations in a predictable manner. Other design goals may also be optimised such as passenger comfort, energy efficiency and traction.
For nearly all wheeled vehicles, when the vehicle accelerates there is an increase in force between the rear wheel(s) and the terrain. This occurs in conjunction with a decrease in force between the front wheel(s) and the terrain. This phenomenon is known in the field of vehicle dynamics as ‘weight transfer’. The opposite occurs when a vehicle decelerates.
For a vehicle having a suspension system, ‘weight transfer’ can have a significant effect on the dynamic behaviour of the vehicle. During weight transfer, a vehicle having a suspension system may exhibit some compression/extension of the suspension system due to the increased/decreased loading that occurs. Typically, as a suspended vehicle accelerates (for example from a stationary position to a moving state), weight transfer causes the rear of the suspended mass to move closer to the ground (‘squat’), while causing the front of the suspended mass to move away from the ground (‘rise’). The opposite occurs when a vehicle decelerates. Typically, during deceleration, weight transfer causes the rear of the suspended mass to ‘rise’, and the front of the suspended mass to ‘dive’.
For a vehicle having a suspension system, it is known that when power is transmitted through the drivetrain, forces are applied to movable elements of the suspension system which can alter its behaviour. Under powered acceleration or braking, a suspension system therefore has forces acting on it due to weight transfer and also due to power transmission through the drivetrain. For a vehicle with a rear driven wheel suspension system, the squat that occurs due to weight transfer under powered acceleration may be counteracted by the forces which are imparted into the suspension system by the drivetrain. In this way, an extension force may be generated in the rear suspension system that can counteract the compression force that occurs due to weight transfer. A vehicle with this characteristic is said to exhibit ‘anti-squat’. Similarly, the rise that occurs due to weight transfer under braking may also be counteracted by the forces which are imparted into the suspension system by the drivetrain. In this way, a compression force may be generated in the rear suspension system that can counteract the extension force that occurs due to weight transfer. A vehicle with this characteristic is said to exhibit ‘anti-rise’.
The dynamic behaviour of the vehicle under powered acceleration and braking is therefore dependent on how much anti-squat and anti-rise the vehicle exhibits throughout its entire range of suspension travel (referred to herein as the acceleration response and braking response respectively). For a front driven wheel suspension, the acceleration response would refer to ‘anti-rise’ and the braking response would refer to ‘anti-dive’.
Acceleration response and braking response are important design considerations for vehicle suspension designers as they directly influence the dynamic feel, handling and performance of the vehicle. There are currently many limitations which a suspension designer faces when trying to design a vehicle suspension system having a desired acceleration response. Often a particular acceleration response cannot be achieved without affecting other important design variables such as the driven wheel path, location of the power source or some other aspect of vehicle geometry that may have been optimised to meet other goals. It would be therefore be advantageous to have a suspension system that allowed the acceleration response to be tuned independently from the existing structural vehicle geometry so that other design variables are not compromised.
Further, the acceleration or braking response able to be achieved with most vehicles is limiting and it may not be possible at all to achieve a desired acceleration or braking response with some suspension systems. It would therefore be advantageous if there was a suspension system that allowed a vehicle to achieve any desired acceleration or braking response that would be practically useful.